3.91 Vs. 4.1 Differential

tsupler · 25th October 2000, 04:09

Not to sound to ignorant, but I've seen several folks looking for a 4.1 diff for their M5's. Mine as I assume most came stock with a 3.91, if memory serves me right some later ones ,after 91, may have come w/3.71. Well anyway could someone explain to me what performance or other gain you get going from 3.91 to 4.1. Is it faster acceleration at expense of top speed or the opposite? Just wondering.

------------------
Tim Supler

KLittle · 25th October 2000, 06:57

Tim:

I had the same question after seeing that some were looking for the 4:10 ratio. I have both the 3:91 and 3:73 (or is it 3:71, not sure) represented in my cars. Personally I feel that the 3:91 is too low and prefer the higher ratio. I do see myself shifting too quickly with the 3:91. It is also easier to bump the rev-limiter with the lower ratio.

I did not buy these cars for their 0-60 performance, nor would I assume that many others have. I personally would not want to sacrifice the top end speed for the gain off the line.

Regards,

Ken Little

'91 M5 Mauritius Blue
'91 M5 Alpine White

Ken Semak · 3rd November 2000, 20:19

It's faster acceleration at the expense of top speed. Also, lower gas mileage and more wear on the engine (turning higher RPM's for a given speed)

Quote:

Originally posted by tsupler:
Not to sound to ignorant, but I've seen several folks looking for a 4.1 diff for their M5's. Mine as I assume most came stock with a 3.91, if memory serves me right some later ones ,after 91, may have come w/3.71. Well anyway could someone explain to me what performance or other gain you get going from 3.91 to 4.1. Is it faster acceleration at expense of top speed or the opposite? Just wondering.

Hattar · 22nd November 2000, 23:24

Quote:

Originally posted by Ken Semak:
It's faster acceleration at the expense of top speed. Also, lower gas mileage and more wear on the engine (turning higher RPM's for a given speed)

I have a european model 1989. I don't know the ratio in this car but it's definitively too low. Guess it is 4.1, maybe somebody can tell me? The car has a "Sport" gear box. I never use the 1'st gear (it also starts in the 3'rd). At 100 km/h the engine speed is ~3 200 rpm in 5'th gear and with the rev limiter at ~7 300 rpm the car can't go faster than 217 km/h(according to the computer, 230 km/h according to the needle). This car should do 250 km/h! Fuel consumption is high, rev limiter is very active and I don't believe that the acceleration is any faster with this kind of differential. You have to start in 2'nd gear and shift gears VERY often. Maybe this differential could work better on a race track? A change with somebodys 3.91 would be nice ...

raymondw · 23rd November 2000, 11:45

Hmmm,

Ommen you say. Be carefull then with the cops there. They are very keen on speeders.

Especially the road from Ommen to Raalte.

With best regards,

Raymond

bmwvsferrari · 18th December 2000, 06:06

A change with somebodys 3.91 would be nice ...

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How is this going on?

Hattar · 2nd January 2001, 02:05

Nothing is really going at the moment but my proposal to change with somebody was (quite) serious.

CarlT · 3rd January 2001, 05:12

Hi Hattar

You should "post" your diff for sale on the www.bimmer.org forum. Changing the diff to a higher ratio seems to be a popular option for many Americans as they often favor acceleration over speed. ( No offense anyone, just facts ) Find out your exact ratio, there is a indication/number on the diff itself. 850 owners might also be interested as its a direct fit.

Regards
Carl

bmwvsferrari · 6th January 2001, 14:53

Quote:

Originally posted by Hattar:
Nothing is really going at the moment but my proposal to change with somebody was (quite) serious.

I just want to swap mine with 3.91 with something like 4.1??
Mine has milage of 54k miles.

Hattar · 14th January 2001, 02:16

I'll keep you in mind if I finally decide to change for something like 3.91. Is it urgent for you? Your address is Honkong...

Mine has milage of 106k miles.